1911 Encyclopædia Britannica/Petalite

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PETALITE, a mineral species consisting of lithium aluminium silicate, LiAl(Si2O5)2. The monoclinic crystals approach spodumene (q.v.) in form, which is also a lithium aluminium silicate with the formula LiAl(SiO3)2. There is a perfect cleavage parallel to the basal plane, and the mineral usually occurs in platy cleavage masses; on this account it was named, from Gr. πἑταλον (a leaf). The hardness is 6½ and the specific gravity 2.4 (that of spodumene being 3.16). The mineral is colourless or occasionally reddish, varies from transparent to translucent, and has a vitreous lustre. It was discovered in 1800 as cleavage masses in an iron mine on the island of Utö in the Stockholm archipelago, where it is associated with lepidolite, tourmaline (rubellite and indicolite) and spodumene. A variety known as “castor” is found as transparent glassy crystals associated with pollux (q.v.) in cavities in the granite of Elba. (L. J. S.)